Inbred mice and rats and T. cruzi clones are being used to develop experimental models for Chagas' disease. The studies confirm the importance of both the natural heterogeneity of the parasite population in chronic chagasic patients and the parasite's role in the course and outcome of a T. cruzi infection. Various complex aspects of human disease can be separated using inbred rats and mice infected with different T. cruzi clones indicating the potential usefulness of this system in developing models for Chagas' disease.